Heavy metals are highly poisonous and also are highly likely to remain in the soil or concentrate in the living body. Therefore, it is necessary to remove them from industrial waste water, service water, environmental water, food products, chemicals, etc., as much as possible. In addition, a large amount of rare metal is contained in discarded electronic devices, which is a precious resource called “urban mine.” Therefore, technologies related to the recovery of valuable metals contained therein have been developed. As a technique for removing heavy metals from a treated liquid, various methods have been practiced, including coagulation sedimentation. As advanced removal/recovery methods, methods using ion-exchange resins and chelating resins have been widely used. Generally, these treated liquids contain high concentrations of salts and organic substances. Thus, the removal of heavy metals with ion-exchange resins is often difficult, and it is said that the removal and recovery can be performed more efficiently using a chelating resin.
A chelating resin is used as a material for the adsorption and recovery of heavy metal elements in a solution containing high concentrations of salts, which is difficult using an ion-exchange resin (see Nonpatent Documents 1 to 4). The ability to form a complex with a metal element differs depending on the functional group structure, and thus chelating resins having various functional groups, such as an iminodiacetic acid (IDA) group, a low-molecular-weight polyamine group, an aminophosphate group, an isothionium group, a dithiocarbamic acid group, and a glucamine group, have been developed (see Patent Document 5). Among them, chelating resins that are commercially available and having introduced thereinto an IDA group having high general versatility are mainly used. However, in the case of an IDA-type chelating resin, because of obstruction by alkali metals or alkaline earth metals, it is often difficult to remove and recover small amounts of metals from a solution having a high salt concentration. In addition, although an IDA-type chelating resin forms a complex with a large number of metals, the stability constant of the formed complex is considerably lower as compared with ethylenediaminetetraacetic acid (EDTA), which is a typical chelating agent. The low stability constant is a major factor of the fluctuation in the removal and recovery due to obstruction by coexistent elements.
It is known that in a chelating agent of a polyaminocarboxylic acid type, such as IDA or EDTA, the stability constant of a complex tends to increase with an increase in the repetition of ethyleneimine (an increase in the chain length) (see Nonpatent Document 6 and Nonpatent Document 7). A chelating resin having an aminocarboxylic-acid-type functional group with an increased chain length has been disclosed. Patent Document 1 discloses a diethylenetriamine-N,N,N′,N′-tetraacetic acid type, which has been carboxymethylated by the introduction of diethylenetriamine, while Patent Document 2 discloses a diethylenetriamine-N,N′,N″,N″-tetraacetic acid type, which has been carboxymethylated by the introduction of diethylenetriamine, etc. Although there is no clear description, it is understood that these chelating resins have a higher stability constant than IDA-type resins. Further, it is expected that by increasing the functional group chain length, the stability constant of a complex can be improved, and also a plurality of metals can be adsorbed into one molecule. Patent Document 3 discloses a chelating resin having a functional group obtained by partially carboxymethylating polyethyleneimine having an average molecular weight of 200 to 600. This chelating resin has adsorption capacity obviously higher than that of an IDA-type chelating resin, and an improvement is seen in stability constant due to an increase in the chain length of the functional group. Another feature of the chelating resin is that it is resistant to obstruction by alkali metals or alkaline earth metals.
Unlike the conventional production method in which a chelating functional group is introduced into a certain carrier, it is also possible to produce a chelating resin by crosslinking a chelating polymer or a polymer capable of forming a chelating functional group. Nonpatent Document 5 also reports a study of producing a chelating resin by crosslinking chitosan, which is a polysaccharide having an amino group. As a crosslinking agent for chitosan, epichlorohydrin, glutaraldehyde, ethylene glycol diglycidyl ether, or the like is used. Nonpatent Document 5 shows the usefulness of chelating resins obtained by crosslinking a chelating polymer, but they are all in the laboratory level. Although chitosan can be easily industrially obtained, it is not necessarily inexpensive. In addition, the acid resistance of chitosan itself is not high. Therefore, there are a large number of problems in application to the actual removal and recovery of metals.
Meanwhile, the form of an adsorbing material is also problematic. A chelating resin is a particulate adsorbing material like activated carbon and ion-exchange resins and has been used in a wide range of fields including a wastewater treatment and a water purification treatment. A water treatment technique using these particulate adsorbing materials has already been established and is expected to be heavily used also in the future. However, because it has a particulate form, such a particulate adsorbing material has to be packed in a specific can and used. Therefore, it may be difficult to adapt to some conditions of use or some installation environments. That is, in order to meet various demands, it is necessary to improve not only the adsorption characteristics of an adsorbing material, but also its form.
In order to solve such a problem, a fibrous chelating adsorbing material that can be easily processed into various forms and can meet various demands has been proposed. Patent Document 4 discloses fibers having a chelating functional group introduced thereinto by a chemical grafting method. Patent Documents 5 and 6 radical formation by radiation exposure and fibers having a chelating functional group introduced thereinto by a graft polymerization method. Patent Documents 7 and 8 disclose chelating fibers obtained by wet blend spinning. These chelating fibers are likely to have sufficient functions and show quick-adsorption characteristics. However, they have some problems in the production method. In a chemical grafting method, the kind of graftable fiber is limited, and also the production process is complicated. A radiation grafting method is advantageous in that it can be applied to various fibers as compared with the chemical grafting method. However, for the handling of radiation, the operation is performed in a specific environment, and thus it cannot be regarded as a simple and inexpensive production method. Meanwhile, according to the wet blend-spinning method disclosed in Patent Documents 7 and 8, a polymer having chelating ability is subjected to wet blend spinning together with viscose, which allows for mass production at low cost using existing facilities. In addition, because a polymeric functional group is used, like the chelating resin having a long-chain functional group shown in Patent Document 3, high adsorption characteristics are shown. However, the production of chelating fibers having different adsorption characteristics has a problem in that a polymer suited to the desired adsorption characteristics has to be synthesized each time.
Patent Document 9 discloses fibers obtained using a method for injecting a low-molecular-weight chelating agent under high-temperature and high-pressure conditions. The chelating agent injection/impregnation method is advantageous in that existing fibers and cloths can be used, and also that the kind of chelating agent can be changed to easily achieve diversification. However, according to the disclosed conditions, a supercritical fluid such as carbon dioxide is the most effective, and also the pressurizing conditions include an extremely high pressure of 100 atm (9.8×106 pa) to 250 atm (2.45×107 pa). Therefore, it cannot be necessarily regarded as a simple production method.